Oil and gas well drilling operations commonly make use of logging while drilling (LWD) sensors to acquire logging data as the well bore is being drilled. This data may provide information about the progress of the drilling operation or the earth formations surrounding the well bore. Significant benefit may be obtained by improved control of downhole sensors from the rig floor or from remote locations. For example, the ability to send commands to downhole sensors that selectively activate the sensors can conserve battery life and thereby increase the amount of downhole time a sensor is useful.
Directional drilling operations are particularly enhanced by improved control. The ability to efficiently and reliably transmit commands from an operator to downhole drilling hardware may enhance the precision of the drilling operation. Downhole drilling hardware that, for example, deflects a portion of the drill string to steer the drilling tool is typically more effective when under tight control by an operator. The ability to continuously adjust the projected direction of the well path by sending commands to a steering tool may enable an operator to fine tune the projected well path based on substantially real-time survey and/or logging data. In such applications, both accuracy and timeliness of data transmission are clearly advantageous.
Prior art communication techniques that rely on the rotation rate of the drill string to encode data are known. For example U.S. Pat. No. 5,603,386 to Webster discloses a method in which the absolute rotation rate of the drill string is utilized to encode steering tool commands. U.S. Pat. No. 7,245,229 to Baron et al discloses a method in which a difference between first and second rotation rates is used to encode steering tool commands. U.S. Pat. No. 7,222,681 to Jones et al discloses a method in which steering tool commands and/or data may be encoded in a sequence of varying drill string rotation rates and drilling fluid flow rates. The varying rotation rates and flow rates are measured downhole and processed to decode the data and/or the commands.
While drill string rotation rate encoding techniques are commercially serviceable, there is room for improvement in certain downhole applications. For example, precise measurement of the drill string rotation rate can become problematic in deep and/or horizontal wells or when stick/slip conditions are encountered. Rotation rate encoding also commonly requires the drilling process to be interrupted and the drill bit to be lifted off bottom. Therefore, there exists a need for an improved downlinking system for downhole tools.